Cesco The aptitude testing done by the organisations you mention is fine - some use the same or similar procedures as GAPAN. I have no doubt that the objective assessment is just as valid as GAPAN's. The advice you receive, as at GAPAN, will depend on the knowledge and independence of the assessor. I would hope that the advice would be entirely uninfluenced by the organisation's position as an FTO and contractor to certain airlines, but forgive me if I'm slightly sceptical about this!
MFWF, if you are self-sponsored and have not undertaken any selection procedure at the start of your ATPL training, you potentially fall into the trap I described earlier. Your military service may have an influence at some companies, particularly if the chief pilot or head of recruiting is ex-military, but I wouldn't put too much weight on that.
As WWW says, the market for the self-sponsored non-selected pilot is still healthy, though the route you may have to take may not involve 737s at 200 hours! Flight instruction is a worthy and worthwhile way of gaining hours and experience, but choose your employer carefully. I have serious doubts that the 'self-improver' route will continue in anything like its present proportion of wannabes in the medium term, but remember I am just speculating. Nothing has changed as yet.
Scroggs